Electrically-initiated or so-called "electric" detonators are commonly employed for actuating one or more explosive devices on various types of well bore tools such as perforating guns, explosive cutting tools, chemical tubing cutters and explosive backoff tools. These tools are typically dependently supported in a well bore by a so-called "wireline" or suspension cable with electrical conductors connected to a surface power source. The detonators that are typically used with these wireline tools with explosive devices are usually comprised of a fluid-tight hollow shell encapsulating an igniter charge (such as black powder or an ignition bead) that is disposed around an electrical bridge wire positioned adjacent to a primer explosive charge such as lead azide that is set off when electric current is passed through the bridge wire. Some detonators may also include a booster charge of a more-powerful, less-sensitive secondary explosive (such as RDX or PETN) which is cooperatively arranged in the shell to be detonated by the less-powerful primer explosive charge. These detonators are typically coupled to an explosive detonating cord positioned in detonating proximity of the one or more explosive charges carried by the wireline tool.
It is, of course, imperative that none of these explosive devices are inadvertently actuated while the well bore tool is at the surface to prevent fatalities and injuries to personnel as well as avoid damaging nearby equipment. One common cause of the inadvertent actuation of wireline well tools employing electric detonators is the careless application of power to the conductors in the cable after the detonator has been electrically connected to the conductors. To minimize that risk, key-operated switches are frequently used for disabling the surface power source until the well tool has been lowered to a safe depth in the well bore. Another common safety technique is to enclose the detonator in a so-called "safety tube" until the detonator is installed in the tool. It must also be realized that should the wireline tool be returned to the surface without its explosive charges having been fired, this significant hazard to nearby personnel and equipment will again reappear while the detonator is being removed from the tool body, disconnected from the detonating cord and the cable conductors, and returned to a safety tube or some other suitable explosion-resistant container.
These safety procedures will, of course, greatly reduce the chances that some human error will be responsible for inadvertent actuation of one of these well tools with explosive devices while it is located at the surface. Nevertheless, a major source of the inadvertent actuation of these typical wireline tools is that the electric detonators commonly used in these tools are quite susceptible to strong electromagnetic fields. Another source of inadvertent actuation of these detonators is the unpredictable presence of so-called "stray voltages" which may sporadically appear in the structural members of the drilling platform. Such stray voltages are not ordinarily present; but these voltages are often created by power generators being used on the drilling rig as well as the cathodic protection systems used to counter galvanic corrosion cells that may be present at various locations in the structure. Lightning may also set off these detonators. At times, hazardous voltage differences may also be developed between the wellhead, the structure of the drilling rig and the electrical equipment used to operate the well tools. A recent SPE technical paper which was authored by K. B. Huber and titled "Safe Perforating Unaffected by Radio and Electric Power" (SPE 20635 presented Sep. 22-26, 1990) gives an analysis of the hazards and the current state of the prior art for safeguarding wireline tools with explosive devices such as various types of perforators.
Because of these potential hazards that exist once a typical wireline explosive tool has been armed, many proposals have been made heretofore for appropriate safeguards and precautions to be taken while these tools are at the surface. For instance, when a perforating gun is being prepared for lowering into a wellbore, in keeping with the susceptibility of typical electric detonators to strong electromagnetic fields it is prudent to maintain strict radio silence in the vicinity. Ordinarily temporary restrictions on nearby radio transmissions will not represent a significant problem on a land rig. On the other hand, when a wireline tool with explosives is being used on a drilling vessel or an offshore platform, it is a common practice to at least restrict, if not totally prohibit, radio and radar transmissions from the platform and any surface vessels and helicopters in the vicinity of the operation. It may be necessary to postpone welding operations on the rig or platform also since welding machines develop currents in the structure that may initiate a sensitive electric detonator in an unprotected explosive tool that is located at the surface.
It will, of course, be recognized that an inordinate amount of time is lost when a wireline explosive tool with an electrical detonator is being prepared for operation on an offshore platform is being prepared since operations unrelated to the particular operation must be curtailed. For example, movements of personnel and equipment by helicopters and surface vessels must be limited to avoid radio and radar transmissions which might set off the detonator. Thus, when an operation with a wireline tool carrying explosives is being considered, the relative priorities of the various operations must be taken into account to decide which of these activities must be curtailed or even suspended in favor of higher-priority tasks. These problems relating to one offshore rig may similarly affect operations on nearby rigs. Accordingly, where there are a large number of these hazardous operations in a limited geographical area, it will be necessary to coordinate the various operations in that field to at least minimize the obvious restrictive effects on those operations.
In view of these problems, various proposals have been made heretofore to disarm these electrical detonators by temporarily interrupting the explosive train between the detonator and the other explosives in the tool. It is, of course, well known that a barrier formed of a dense substance, such as a rubber or metal plug, positioned between the donor and receptor charges in a typical detonator will attenuate the detonation forces of the donor explosive sufficiently to reliably block the detonation of the receptor charge. For example, some commercial detonators are sold with rubber plugs disposed in the fluid-disabling ports that communicate to the empty space between the adjacent charges. This same principle is, of course, employed with the barriers that are disclosed in U.S. Pat. No. 4,314,614 as well as in FIG. 7 of U.S. Pat. No. 4,011,815. U.S. Pat. No. 4,523,650 discloses a disarming device employing a rotatable barrier that is initially positioned to interpose a solid detonation-blocking wall between the donor and receptor explosives in the detonator until just before the perforator is ready to be lowered into the well bore. To arm that detonator, the barrier is rotated to align a booster explosive in the barrier with the spatially-arranged donor and receptor explosives. With any of these prior-art safearming devices, it is, of course, essential to either completely remove or else reposition the temporary barrier before the perforator is lowered into a well bore so that it will thereafter be free to operate. Once any of these temporary barriers has been removed from the perforating gun or repositioned, the detonator in the perforator is thereafter subject to being inadvertently detonated by any of the extraneous hazards discussed above.
A new electronic detonating system described in the above-identified SPE paper includes an electrically-actuated initiator assembly which includes an encapsulated pellet of a secondary explosive that is disposed around a foil-covered metallic bridge. The initiator assembly is spatially disposed from a secondary explosive booster and isolated therefrom by a thin wall or metal partition. The initiator assembly is initially disarmed by means of a removable safety barrier which is temporarily placed in the space between the two charges until the perforator is ready to be lowered into a well bore. The detonating system further includes an electronic cartridge arranged for supplying a sudden burst of electrical energy to the foil-covered bridge to instantaneously vaporize the bridge for forcibly driving a portion of the foil bridge against the secondary explosive pellet with sufficient force to set off the pellet. The detonation of this secondary pellet will, in turn, cause a plug or so-called "flyer" to be sheared out of the end partition of the initiator assembly and forcibly driven across the space between the charges to strike the adjacent end of the secondary explosive booster charge with sufficient force to sequentially induce high-order detonations of the booster charge and a detonating cord that is coupled thereto. It will, of course, be appreciated that since this detonating system does not have any primary explosives, this system is not as suseptible to extraneous electrical energy as are the other prior-art detonating systems described above. Nevertheless, it must be recognized that since an electronic detonating system of this nature is quite expensive, cost considerations may restrict the use of these systems to perforating operations in high-risk locations.
One of the most important advances that has been recently made for selectively and inexpensively safeguarding wireline tools carrying explosive charges has been to form barriers of low-temperature fusible metal alloys and permanently install one of them between the donor and receptor explosives in a detonator for reliably blocking the transmission of detonation forces from the donor explosive until the detonator has been subjected to well bore temperatures greater than the established melting point of the alloy. This unique concept is explained in a co-pending application Ser. No. 550,862 which was filed Jul. 10, 1990, by the Applicants of the present application and is now U.S. Pat. No. 5,070,788. These low-temperature fusible alloy barriers are, however, not feasible in wells where the well bore temperatures are about the same as the lowest attainable melting points of these eutectic and non-eutectic metals.